1. Field of the Invention
A prior art scanning transmission electron microscope has aperture stops to control the emission current and illumination angle of the electron beam relative to a specimen. The aperture stops are movably mounted near the principal plane of a condenser lens (brightness-adjusting lens) immediately behind the electron source. Usually, plural aperture stops having different hole diameters are used for the above-described control operations. The aperture stops are mounted to a stop holder having a moving mechanism. Because the aperture stops are close to the principal plane of the condenser lens, if the strength of the condenser lens is varied while one aperture stop is in operation, the total amount of current impinging on the specimen does not vary so much.
2. Description of Related Art
In the case of an electron microscope employing no aberration correction technique, the necessity to controllably vary the angular aperture of the electron beam in increments of a few mrad is low because no aberration correction is made. Accordingly, it is customary to prepare aperture stops having hole diameters which are roughly doubled successively, such as 20, 40, 70, 100, and 200 μm.
A conventional scanning transmission electron microscope utilizing an aberration correction technique is disclosed in JP-A-2007-173132. In this known microscope, an electron source, a condenser lens, condenser apertures (aperture stops), a spherical aberration corrector, a deflector, a transfer lens, and an objective lens are arranged in this order from the upstream side. Also, in this case, there are condenser apertures having different hole diameters. An electron beam is made to pass through a selected one of these apertures, thus varying the angular aperture. Which of the aperture stops is selected depends on the balance between the spherical aberration and diffraction aberration at a desired magnification. Then, the beam is suppressed in spherical aberration by an aberration corrector and made to impinge on the specimen.
Another scanning transmission electron microscope utilizing an aberration correction technique is disclosed in JP-A-2007-95335. In this known instrument, two transfer lens subassemblies giving a magnification M of 1 or more are disposed between a spherical aberration corrector and an objective lens. The spherical aberration corrector produces a negative spherical aberration that cancels out the positive spherical aberration of the objective lens. However, third-order star aberration S3 and third-order four-fold astigmatism A3 which occur concomitantly can no longer be neglected. Accordingly, in the technique of JPA-2007-95335, the spherical aberration appearing on the specimen is canceled out and the effects of the third-order star aberration S3 and third-order four-fold astigmatism A3 are reduced by adjusting the bore diameter of the spherical aberration corrector and setting the magnification M of the transfer lens subassemblies to 1 or more.
In order to achieve aberration correction during high magnification imaging, it is necessary to set the aperture value appropriately for the residual aberrations. That is, the angular aperture of the electron beam needs to be adjusted in small increments, e.g., the semi-angular aperture is varied in increments of 2 mrad about the angle of 30 mrad. Therefore, with aperture stops differing greatly in hole diameter as described previously, it is difficult to adjust the angular aperture minutely. Furthermore, the present situation in conventional scanning transmission electron microscopes equipped with an aberration corrector is that the angular aperture is varied by selecting one aperture stop and adjusting the strength of each lens without varying the selected aperture stop. Therefore, where the angular aperture is adjusted minutely by adjusting the strength of each lens, correction conditions for the aberrations are also required to be readjusted. This complicates the adjustment of the whole electron optical system.